Fix refreshing the volume list when the volume is unmounted.

When a volume is unmounted, while it was selected, then the list model is
updated, and since there is no selection anymore, the current entry from the
directory model is selected. However, the corresponding VolumeInfo is gone
so we can't call resolveRootEntry().

This patch adds a check. If the volume is gone, then the selection is aborted.

TEST=Tested manually with FSP.
BUG=418513

Review URL: https://codereview.chromium.org/595903003

Cr-Commit-Position: refs/heads/master@{#297188}

ui/file_manager/file_manager/foreground/js/directory_tree.js

@@ -1018,6 +1018,8 @@ DirectoryTree.prototype.selectByEntry = function(entry) {
   this.updateSubDirectories(false /* recursive */);
   var currentSequence = ++this.sequence_;
   var volumeInfo = this.volumeManager_.getVolumeInfo(entry);
+  if (!volumeInfo)
+    return;
   volumeInfo.resolveDisplayRoot(function() {
     if (this.sequence_ !== currentSequence)
       return;